1. Field of the Invention
The present invention relates to a switch having a switching mechanism, which switches at a response temperature, for opening and closing a circuit that can be connected to external terminals of the switch, such that the switching mechanism comprises a movable contact, electrically connected to one external terminal, which as a function of the temperature of a bimetallic part is in contact with a fixed contact that is electrically connected to the other external terminal.
2. Related Prior Art
Switches of this kind, which are also referred to as temperature controllers, are generally known.
A basic distinction may be made between two different design variants of such switches. With the first configuration, the movable contact is held by a spring element that is clamped at one end; while with the second configuration, the movable contact is arranged approximately centeredly on a spring snap disk which is placed unconstrainedly into a housing and is braced with its rim against a housing part of the switch.
When the switch is incorporated with its external terminals into a circuit, with the switch closed current then flows via the one external terminal, through the fixed and movable contacts and the spring element, to the other external terminal. The spring element can be made either from a bimetallic or from spring steel.
If the spring element is made of a bimetallic, the result is a physically very simple design which nevertheless may have the disadvantage that the switching properties of the bimetallic can change due to internal electrical heating. This is prevented if a separate bimetallic part is provided which works against the force of the spring element.
Switches of this kind can be configured as normally-closed or normally-open switches; above its response temperature, the bimetallic part then either pushes the movable contact away from the fixed contact, or brings it into contact therewith.
It is also known to associate series and/or parallel resistances with such switches in order to attain further switching properties. A resistance connected in parallel imparts a self-hold function to a switch which opens in response to over-temperature: the bimetallic part is held at a temperature above its response temperature (and thus in the open state) by the resistive heat generated in the resistance.
A series resistance additionally imparts a current sensitivity to a switch equipped therewith: the current flowing through the switch heats the series resistance, such that the resistive heat generated therein causes the temperature of the bimetallic part to be elevated above the response temperature (and thus causes the switch to open) if the flowing current has exceeded a certain limit value.
Switches of this kind are, for example, connected in series with a load being protected, so as to protect the latter from over-temperature and/or overcurrent. They are utilized in particular to protect electric motors, transformers, household electrical appliances, etc.
It is known that the switching behavior of the bimetallic part changes over time: as a result of age, the switching temperature shifts upward by as much as 30 degrees C., which can lead to safety problems. In order to eliminate this problem, it is already known to connect, in series with a switch of this kind, a separate safety fuse which opens above the response temperature of the switch but below the maximum allowable temperature. Safety fuses of this kind are used in particular in transformers, where temperature controllers without self-holding are used only in conjunction with a safety fuse that is connected in series.
If the switch does not switch at the desired response temperature due to aging of the bimetallic part or some other defect, the temperature of the device which is being monitored, and with which the switch is thermally connected, rises until the melting temperature of the safety fuse is reached. The safety fuse then opens, so that the circuit powering the electrical load being protected is irreversibly interrupted.
The additional use of a safety fuse of this kind entails, however, a number of disadvantages. Firstly, two separate components must be installed on the device being protected, which not only increases the time required for assembly but also requires additional installation space. This is particularly disadvantageous because known safety fuses are very bulky, so that appropriate space on the device being protected must be provided.